Flywheels have emerged as a very attractive energy storage technology for electrical applications, such as uninterruptible power supplies, utility load leveling systems, alternative energy generation, satellites and electric vehicles. Flywheel systems convert back and forth between the rotational energy of a spinning flywheel and electrical energy. A flywheel energy storage system includes a flywheel, a motor and generator, a bearing system and a vacuum enclosure. The rotating flywheel stores the energy mechanically; the motor and generator converts between electrical and mechanical, while the bearing system physically supports the rotating flywheel. High-speed flywheels are normally contained in a vacuum or low-pressure enclosure to minimize aerodynamic losses that would occur from atmospheric operation.
Some of the benefits of flywheel energy storage systems over conventional batteries are longer life and higher reliability. A key component to achieving long life with flywheel energy storage systems is the bearing system. Flywheels have been supported by numerous configurations of bearings that have included magnetic, mechanical and fluid type. Not only must the bearing system be capable of long life operation, but it must also allow smooth operation, have low friction and in many cases be very low in cost.
Accordingly, the invention is a flywheel energy storage system with an arrangement of magnetic and mechanical bearings that provides passive, reliable, long life operation with very low cost. The flywheel system is comprised of a flywheel inside an enclosed low-pressure chamber for reduction of aerodynamic losses. An attached motor/generator accelerates and decelerates the flywheel for storing and retrieving energy. The flywheel is supported for rotation about a vertical axis on the combination bearing system. At one axial end of the flywheel is a magnetic bearing that provides both axial and radial centering support. The passive radial magnetic bearing carries the majority of the flywheel""s weight and also provides low stiffness radial support at that axial end. The opposite axial end of the flywheel is supported on a mechanical rolling element bearing, such as a ball bearing set. The rolling element bearing stabilizes the axial direction support of the flywheel and accomplishes this with a well-established and long life type of mechanical bearing. It also stabilizes the tilting of the flywheel by providing some radial support.
Although rolling element bearings are more costly than pivot or pin type bearings employed in previous systems, the life and reliability of flywheel energy storage systems with an arrangement of magnetic and mechanical bearings in accordance with the invention is improved. The flywheel is connected to the rolling element bearing with a connecting element that imparts a low radial stiffness. In one embodiment of the invention, the connecting element is a quill shaft. Another embodiment uses a low stiffness radial spring between the flywheel and the rolling element bearing. The benefit of the low radial stiffness from both the passive radial magnetic bearing and the mechanical bearing support is that the flywheel can operate above its rigid body critical speeds. The flywheel quickly accelerates to supercritical operation where the flywheel spins about its mass center and bearing loads and vibrations are greatly reduced while mechanical bearing life is increased. Compared with rigidly supported flywheel systems, the invention can operate at high speeds and have longer life. Because the axial stabilization for the flywheel support is imparted from a mechanical bearing located at a only single axial end of the flywheel, Poisson Ratio or thermal changes in the flywheel length do not result in excessive mechanical bearing loads. The magnetic bearing end of the flywheel simply experiences a slight change in the relative positions between the flywheel and stationary portions. The flywheel bearing arrangement of the invention provides a passive, low cost support for high-speed operation. The magnetic bearing carrying the majority of the flywheel weight significantly extends the life of the mechanical bearing. Compared with fluid type bearings, the invention also exhibits minimal outgassing.
In another embodiment of the invention, passive radial magnetic bearings are employed at both axial ends of the flywheel. In this configuration, the magnetic bearings cooperate to provide unstable axial support but stable radial and stable tilting support. The radial centering stiffness of a single magnetic bearing at one axial end of the flywheel overcomes the unstable tilting moment of the other magnetic bearing at the other axial end. A rolling element mechanical bearing with a low radial stiffness connecting element provides axial stabilization by providing some axial support. With the use of both magnetic bearings, the mechanical bearing need not carry radial loads and the connecting element is preferably made to impart a very low radial stiffness, less than the radial stiffness of the adjacent passive radial magnetic bearing. The life of the rolling element bearing is even further increased.
In several configurations of the invention a shipping and handling mechanism is provided to make the flywheel system more durable and robust. Although the rolling element bearing provides axial support for the flywheel, during transportation of the system, the bearing or quill shaft can become easily damaged especially if the flywheel is of significant weight. The mechanical bearing therefore only provides support in one axial direction. For example, the connection between the flywheel and the mechanical bearing has a stop to prevent motion of the flywheel axially only one way. This allows the flywheel to be stably supported with the magnetic bearing or bearings, and allows the flywheel to slide vertically in the other direction. When the flywheel system is designed such that the magnetic bearings lift more than the weight of the flywheel and the mechanical bearing exerts a downward force in operation, the flywheel can eliminate damage from impacts during handling. When the flywheel system is set down, instead of the large mass of the flywheel impact loading the delicate mechanical bearing, the flywheel simply slides down axial until contacting a stop. The magnetic bearing subsequently pulls the flywheel back upward to engage the mechanical bearing for operation.
In other embodiments of the invention, mechanical rolling element bearings are described for long life operation. Multiple preloaded angular contact bearings as well as parallel bearings can be used to increase life through reduction of carried load. A series bearing arrangement can also be used to increase life by reducing the number of cycles and by limiting the operating speed. Such an arrangement can be beneficially used with dry lubricated bearing sets that are very low outgassing but are speed limited.